The present invention relates generally to a new technique for providing inexpensive, low-noise integrated circuits, and more particularly to an improved technique for providing low noise SiCr thin film integrated circuit resistors without increasing integrated circuit processing costs.
FIG. 1 illustrates a conventional noise testing circuit arrangement 1 for measuring the amount of noise produced by a circuit, which in this case is low-noise integrated circuit operational amplifier 2 including thin film resistors. The (−) input of operational amplifier 2 is connected between the junction between a gain resistor R3 and a feedback resistor R4. The (+) input is coupled to ground by a switch 3 and a resistor R1. The output of operational amplifier 2 is connected to an oscilloscope 5 which is used to determine the noise signal on the output of operational amplifier 2.
SiCr (sichrome) thin film integrated circuit resistors are well known to have various advantages over NiCr thin-film integrated circuit resistors. For conventional NiCr (nichrome) resistors, aluminum interconnect metallization can be formed directly on the resistor contact areas. For manufacture of SiCr resistors, a TiW barrier layer ordinarily is provided between the SiCr contact areas and the aluminum metallization to prevent the aluminum from reacting with the silicon in SiCr.
It is well known that SiCr thin film resistors have certain advantages over standard NiCr thin film resistors. The sheet resistance of a deposited SiCr layer is much more stable than deposited NiCr layer. Consequently, the resistance of NiCr thin-film resistors undergoes substantial changes as further manufacturing steps are performed, especially during packaging of the integrated circuit chip. For example, the resistance of a typical NiCr thin-film resistor might change by several hundred parts per million between the time the NiCr layer is deposited on the wafer and the time at which the packaging of a chip of the wafer has been completed. In contrast, the corresponding change in resistance of a SiCr resistor might be only a few parts per million. It is also well known that SiCr thin film resistors maintain absolute values of the resistance, and therefore maintain nearly constant ratio matching (i.e., the ratio of the resistance of one resistor to the resistance of another resistor is nearly constant), which is far better than the ratio matching achieved by use of conventional NiCr thin film resistors.
The term “high-grade yield” is used to refer to a yield of an integrated circuit with respect to one or more particular parameters of the integrated circuit which are regarded as critical to its performance. For example, if the critical parameter of the integrated circuit does not shift by more than a specified amount over a particular amount of time or a particular sequence of manufacturing steps, the integrated circuit is considered to have a “high-grade yield” and therefore be a “high-grade chip”. Otherwise, the integrated circuit is considered to have a “low-grade yield” and therefore to be a “low-grade chip”.
SiCr thin film resistors generally have a substantially lower TCR (temperature coefficient of resistance) than NiCr thin film resistors. Generally, deposited NiCr oxidizes, and the process is somewhat uncontrolled because the oxidation of the NiCr causes its sheet resistance to shift as much as 20% or more during subsequent processing steps. Consequently, annealing or “bake” cycles generally are required to adjust the sheet resistance of deposited NiCr layers to desired “target” values. In contrast, the process of depositing SiCr is much more precisely controlled, because the SiCr does not react with oxygen or other ambient gases and therefore maintains a nearly constant sheet resistance during subsequent wafer processing and chip packaging operations. Consequently, time-consuming, costly subsequent annealing cycles are not required for the purpose of adjusting the sheet resistance, and only a single annealing cycle is required for the purpose of achieving sheet resistance stability. Therefore, the overall cost of integrated circuits having SiCr thin film resistors is substantially less than that of integrated circuits having NiCr resistors.
Published UK patent application GB 224-0875A published Aug. 14, 1991 entitled “A Method for Forming a Thin Film Resistor on an IC Wafer” by William Allen Lane and Andrew David Bain, assigned to Analog Research and Developments Ltd. in Ireland, incorporated herein by reference, discloses use of a TiW layer 15,18 between a contact area of a SiCr thin film resistor 4 and an aluminum metal conductor 9.
Referring to FIG. 2, it has become conventional in the integrated circuit industry to provide a TiW “barrier” layer 13 between an aluminum metallization layer 14 and the contact area 12A of a SiCr thin film resistor 12. The TiW barrier layer 14 is provided mainly for the purpose of preventing a reaction between the aluminum and the silicon present in the SiCr. However, the diagram of FIG. 2 does not show a very thin, irregular oxide between the TiW barrier layer 14 and the SiCr thin film resistive layer 12.
The noise problem to which the invention relates was discovered by using the test arrangement shown in FIG. 1 to first measure the noise in a conventional low-noise operational amplifier test circuit 2 having NiCr thin film resistors fabricated using a conventional NiCr process. The resulting measured current noise level was very low, only 19.1 Pico amperes peak-to-peak.
Next, the same test arrangement shown in FIG. 1 was used to measure the noise in another identical low-noise operational amplifier having its thin film resistors composed of SiCr instead of NiCr. The measured current noise level for SiCr thin film resistors was 50.3 picoamperes peak-to-peak, nearly 3 times higher than for NiCr resistors. The noise was observed as broadband noise in the 0.1 to 10 Hz range.
Thus, there is an unmet need for a way of manufacturing integrated circuits including SiCr thin film resistors which do not generate excessive noise.
There also is an unmet need for a low cost a way of manufacturing integrated circuits including SiCr thin film resistors which do not generate excessive noise.